Storage amplifier screen

ABSTRACT

A storage amplifier screen comprising a layer of photoconductive material responsive to an input radiation and a layer of electroluminescent material emissive of an output radiation are sandwiched between two electrical conductive electrodes. The screen provides high sensitivity, resolution and gain. The photoconductive material, doped zinc oxide, provides the property of persistence of the input image under excitation for over 10 minutes after removal of input radiation. The photoconductive layer also provides the property of storage of the image for days in the absence of an electrical field.

United States Patent [1 1 Szepesi 1 1 Jan. 16, 1973 1 STORAGE AMPLIFIERSCREEN [75] Inventor: Zoltan P. J. Szepesl, Elmira, NY.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Nov. 20, 1970 [21] Appl. No.: 91,254

[52] US. Cl. ..250/213 R, 313/108 A [51] Int. Cl ..H0lj 39/12 [58] Fieldof Search..250/213 R, 213 VT, 211 R, 211

.l, 250/71; 317/235 N; 315/169 TV; 338/17; 313/108 R, 108 A, 108 B, 94

[56] References Cited UNITED STATES PATENTS 2/1961 Thornton........250/213 R 3/1970 Burns ..250/211 R Primary Examiner-James W. LawrenceAssistant Examiner-D. C. Nelms Attorney-E. H. Henson and C. F. Renz [57]ABSTRACT A storage amplifier screen comprising a layer ofphotoconductive material responsive to an input radiation and a layer ofelectroluminescent material emissive of an output radiation aresandwiched between two electrical conductive electrodes. The screenprovides high sensitivity, resolution and gain. The photoconductivematerial, doped zinc oxide, provides the property of persistence of theinput image under excitation for over 10 minutes after removal of inputradiation. The photoconductive layer also provides the property ofstorage of the image for days in the absence of an electrical field,

12 Claims, 1 Drawing Figure '8 ,e e l l an PATENTEUJAN 16 I973 3.711.719

WITNESSES; INVENTOR 74% %/M Zolton P. J. Szepesi STORAGE AMPLIFIERSCREEN BACKGROUND OF THE INVENTION This invention is generally directedto a storage amplifier screen and more particularly to a storage screenexhibiting the property of converting radiation in the X-ray region intoa visible radiation.

Several constructions of solid state amplifiers and converters have beendemonstrated over the last 20 years. The principal goal in most of theseconstructions was to build a screen with higher brightness than that ofthe fluoroscopic screen provided in X-ray applications to limit exposureof a patient and permit study by the doctor. Some of these constructionsexhibited storage. However, the storage time was short and usually lessthan a minute. The image deteriorated very much in this small amount oftime. These constructions were of many types and suffered from manydisadvantages. They were limited in resolution, gain, sensitivity andcontrast. Several of the devices were also critical in theirconstruction.

Photographic film is presently used in medical and industrialradiography where high contrast and high resolution is required. Theexpense, the time delay and inconvenience resulting from the requiredprocessing of the film, are undesired features of photographicradiography. A general discussion of this background is found in anarticle entitled Solid State Image Intensifiers, Radiography Amplifiers,and Infrared Converters by the inventor in the Dec. 1969 issue ofElectro- Technology.

SUMMARY OF THE INVENTION This invention is directed to a storage displaypanel incorporating an electroluminescent layer and a photoconductivelayer sensitive to an input radiation image to provide a conductivityimage corresponding to said input image. The photoconductive layer alsoexhibits the property of retaining this conductivity image for asubstantial time after removal of the incident radiation. A suitablydoped zinc oxide layer provides a good quality image for a period oftime of from to 100 minutes and the conductivity image may be erased byheating the storage display panel.

BRIEF DESCRIPTION OF THE DRAWING The invention will become more readilyapparent from the following exemplary description in connection with theaccompanying drawings in which a schematic showing is made of a storagepanel in accordance with the teachings of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT I Referring in detail to thedrawing, a storage panel 10 is illustrated. The storage panel 10 may berectangular in shape and may be 8 X 10 inches in size. The storage panel10 is comprised of a support plate 12 of the suitable materialtransmissive to radiations generated by an electroluminescent layer 14.A suitable material for the plate 12 is glass or plastic. A glass sheet12 having a thickness of about 1.5 millimeters may be utilized. Anelectrical conductive layer 16 is provided on one surface of the supportplate 12 and is also transmissive to radiations from theelectroluminescent layer 14. A

suitable material for the layer 16 is tin oxide evaporated to athickness of about nanometers and having a resistance of about 100 ohmsper square. Electrical conductive bus bars 18 are provided on oppositesides of the conductive coating 16 and these bus bars 18 are of asuitable material such as copper and are connected to a potential source20 through a variable resistance 22. The source 20 may be a AC potentialsource of about 50 volts to provide means of applying electrical currentthrough the layer 16 in order to provide heat to the storage amplifier.The electroluminescent layer 14 is deposited on the conductive layer 16and is comprised of an electroluminescent powder embedded in a highdielectric plastic material with the phosphor to dielectric ratio beingapproximately 3 to l by weight. The thickness of the layer 14 may befrom about 25 to 50 micrometers. The electroluminescent material inlayer 16 may be of a suitable electroluminescent phosphor such as zincsulfide activated with copper and bromine and embedded in a highdielectric constant type plastic such as a mixture of cyanoethyl starchand cyanoethyl sucrose.

A photoconductive layer 23 is provided on the electroluminescent layer14. The layer 23 may be prepared in the following manner: Mix with 100grams of very pure, very fine (less than 1) micron zinc oxide powder, 25milligrams to 1 gram with an optimum amount of I00 milligrams of reagentquality sodium sulfate (Na SO and from 0 to 1 gram optimum 100milligrams of reagent quality lead chloride (PbCl powder. This mixture,which includes less than 2 percent by weight of sodium sulfate and leadchloride, is baked in an air atmosphere at about 1000C for about 2hours. This temperature may range from 900 to [200C After the powder iscooled, it is placed in a blender with deionized water and blended forone minute at high speed. This mixture is then poured into a flask andthe powder is settled down and the water decanted off. This process maybe repeated for washing the powder. The powder is then rinsed with 2-propanol, settled, decanted and ultrasonically agitated for threeminutes. The resulting powder propanol paste is then dried on a 0.8'micrometer pore size filter and then in a forced air oven at a C forabout 30 minutes. The powder then may be passed through a 200 meshsieve.

The above doped zinc oxide powder is then mixed with a plastic about 100grams of the sensitized zinc oxide powder to 3 to 30 grams of a siliconeresin DC 804 purchased from Dow Corning, 25 grams of amyl alcohol and2.5 grams of diethyl carbitol. This is ball mill mixed and is then readyto be applied to the electroluminescent layer 14. The zinc oxidesilicone mixture is bladed onto the electroluminescent layer 14 to athickness of about 200 to 400 micrometers. This coating 23 may beapplied by simply utilizing a blade or a thin layer coating machine. Thebladed-on layer 23 is then allowed to dry in a horizontal position. Thelayer 23 has a resistance of about 10 ohm cm.

An electrically conductive layer 24 is then evaporated onto thephotoconductive layer 23. The layer 24 is transmissive to the inputradiations and a suitable layer for X-rays is gold. The layer 24 alsoabsorbs the disturbing or scattered radiation. The layer 24 isevaporated to a thickness of about 0.1 to l micrometer and has aresistance of about 20 ohms per square.

An AC voltage source 26 is connected between the conductive. layer 16and the layer 24 to provide a voltage of about 200 to 400 volts and of afrequency of about 60to 1000 Hz.

In the operation of the device, the X-ray image is directed onto thestorage amplifier 10 through the layer 24 and modifies the conductivityof the photoconductive layer 23 in accordance with the X-ray imageinput. During thisphase of the operation, neither the potential source20 nor the potential source 26 need be connected to the amplifier. Thestorage layer 23 exhibits the property of integration over a period oftime. After the completion of the exposure by the X-rays, an increasedconductivity image remains in the layer 23 corresponding to the inputradiation. The potential source 26 may be applied, and an image may beviewed through the layer of 20 due to the AC potential applied acrossthe electroluminescent layer 14 and the photoconductive layer 23. Thelight image will correspond to the conductivity image within thephotoconductive layer 23. The image may be viewed in this manner forseveral minutes and for as long as a 100 minutes without a substantialdegradation of the light image. It is also possible, if desired, toremove the excitation of the potential 26 and the conductivity imagewill be retained in the photoconductive layer 23 for a period of severaldays and then by application of potential by the voltage source 26 theimage may be viewed again. Prior to the reuse of the screen, the imagemay be erased by applying the potential source 20 which provides anelectrical current through the layer 16 causing the heating of thephotoconductive layer 23 and thereby removing the conductivity imagefrom the layer 23. After this erasing, the storage amplifier is ready tobe utilized in the manner above. The image can also be erased by simplybaking the entire panel in a furnace for to minutes at a temperature ofabout 100C.

If desired, the device can be made more sensitive to other radiationssuch as visible light by the addition of an organic sensitizing dye tothe zinc oxide powder. For example, by mixing rhodamine, an organic dye,to the zinc oxide powder in a proportion by weight of l to 2000, theamplifier may be made sensitive to the visible light input. t

The required characteristics for the photoconductive layer 23 is amaterial that has a trap density above the Fermi level (shallow traps)higher than 10 traps cm in an energy slice kT for an energy band ofseveral tenths of an electron volt. The materials must have a very lowcapture cross section of deep traps (recombination centers) or extremelyshort hole life time. Materials must have a high bandgap (between thevalence and conduction band). In such material, the available electronswill supply a slowly decaying photoconductive current for severalminutes after their radiation to the storage member is ceased. Thephotoconductive layer 23, after a long time in the dark or after eraser,has a very low dark current. Its shallow traps are empty and noelectrons are thermally excited from the valence band to the conductionband because of the high bandgap (greater, than 3 ev). Irradiation ofthe photoconductor excites electrons from the valence band to theconduction band. These electrons will fill up the shallow traps. Theelectrons in the conduction band and in the shallow traps are in thermalequilibrium; they are changing place many times before recombinationoccurs (electrons extracted by the positive electrode will bereplenished through the ohmic contact of the other electrode). After theexcitation stops, the large number of trapped electrons will support adecaying flow of current until the traps are again emptied.

The storage amplifier described above exhibits along storage time orpersistence and decays to about onethird of maximum brightness in 5 to50 minutes. The device also exhibits high resolution of about 6 to 10line pairs per millimeter. The device also has a high contrastsensitivity and the device also exhibits medium high sensitivity toX-rays and very low sensitivity to visible light. The radiographicquality of the panel is 2-2T. (This means 2T holes of a 2 per centpenetrameter can be detected.) These characteristics are particularlydesirable in that these storage amplifiers may be substituted forradiographic film for nondestructive X-ray testing with the advantage ofimmediate viewing without film processing. The device also may be erasedand utilized over and over.

I claim:

1. A display storage panel for reproduction of an image of incidentradiation comprising a radiation responsive layer comprising zinc oxidematerial containing predetermined amounts of oxygen and sodium, saidradiation responsive layer exhibiting the property of producing aconductivity image corresponding to said incident radiation in responseto said input radiation and persisting after removal of said inputradiation, a display layer of material coupled to said radiationresponsive layer and including material responsive to conductivitychanges in said radiation responsive layer to display an imagecorresponding to said conductivity image.

2. The display storage panel of claim 1 in which said zinc oxidematerial additionally contains predetermined amounts of lead andchlorine.

3. The display storage panel of claim 1 in which said display layer iscomprised of electroluminescent material.

4. The display storage panel of claim 1 in which a plastic materialsurrounds said zinc oxide material.

5. The display storage panel of claim 3 in which electrically conductiveelectrodes are provided for impressing an electric field across saidradiation responsive layer and said delay layer.

6. The display storage panel of claim 5 in which said electric field isprovided by an alternating vcurrent source.

7. The display storage panel of claim 6 in which said input radiationisin the X-ray region.

8. The display storage panel of claim 1 in which said zinc oxidematerial contains dye material to modify the wavelength response of saidradiation responsive layer to said input radiations.

9. The display storage panel of claim 1 in which means is provided forimpressing heat on said radiation responsive layer to remove saidconductivity image from said radiation responsive layer.

10. The display storage panel of claim 9 in which said means forimpressing heat is an electrically conductive electrode extending acrosssaid panel and supplied with electrical current to heat said electrode.

1 1. The display storage panel of claim 4 in which said radiationresponsive layer has a thickness at least four times greater than thethickness of said display layer.

12. The display storage panel of claim 11, in which said radiationresponsive layer is greater than 200 5 micrometers in thickness.

2. The display storage panel of claim 1 in which said zinc oxidematerial additionally contains predetermined amounts of lead andchlorine.
 3. The display storage panel of claim 1 in which said displaylayer is comprised of electroluminescent material.
 4. The displaystorage panel of claim 1 in which a plastic material surrounds said zincoxide material.
 5. The display storage panel of claim 3 in whichelectrically conductive electrodes are provided for impressing anelectric field across said radiation responsive layer and said delaylayer.
 6. The display storage panel of claim 5 in which said electricfield is provided by an alternating current source.
 7. The displaystorage panel of claim 6 in which said input radiation is in the X-rayregion.
 8. The display storage panel of claim 1 in which said zinc oxidematerial contains dye material to modify the wavelength response of saidradiation responsive layer to said input radiations.
 9. The displaystorage panel of claim 1 in which means is provided for impressing heaton said radiation responsive layer to remove said conductivity imagefrom said radiation responsive layer.
 10. The display storage panel ofclaim 9 in which said means for impressing heat is an electricallyconductive electrode extending across said panel and supplied withelectrical current to heat said electrode.
 11. The display storage panelof claim 4 in which said radiation responsive layer has a thickness atleast four times greater than the thickness of said display layer. 12.The display storage panel of claim 11, in which said radiationresponsive layer is greater than 200 micrometers in thickness.